In general, retreading pneumatic tires by applying a precured (e.g. prevulcanized) tread strip to a tire casing is known. Examples of such retreading process, sometimes referred to as cold vulcanizing or cold bond retreading, are disclosed in U.S. Pat. Nos. 2,976,910 issued to Nowak on Mar. 28, 1961; 3,136,673 issued to Carver on Jun. 9, 1964; 3,236,709 issued to Carver on Feb. 22, 1966; 3,283,795 issued to Schelkmann on Nov. 8, 1966; 3,325,326 issued to Schelkmann on Jun. 13, 1967; 3,745,084 issued to Schelkmann on Jul. 10, 1973; 3,752,726 issued to Barefoot on Aug. 14, 1973; 3,951,720 issued to Brodie on Apr. 20, 1976; 4,046,947 issued to Brodie on Sep. 6, 1977; and 4,075,047 issued to Brodie et al. on Feb. 21, 1978; 4,151,027 issued to Schelkmann et al. on Apr. 24, 1979; and, 4,434,018 issued to Brewer on Feb. 28, 1984.
In such retreading procedures, the tire casing to be retreaded is typically buffed to remove excess rubber, to provide a generally evenly textured crown for receiving the cured tread strip and to provide a predetermined shoulder profile. Tire casings typically include a belt package, (a package of steel belts or cables), underlying the road engaging surface, (e.g., the original tread) of the tire. The casing typically is buffed at a predetermined characteristic crown radius generally corresponding to the upper contour of the belt package, but leaving only a predetermined thickness, e.g., 3/32 of an inch, of material remaining over the top belt. The shoulder of the casing is also buffed (trimmed) to eliminate or reduce voids or patterns in the shoulder created by the original tread, and to provide a relatively straight profile between the casing sidewalls and crown. The casing from each of the various types of known new tires has a characteristic crown width, crown buffing radius and shoulder trim angle.
The precured tread strip, typically of a width corresponding to the width of the crown of the casing and cut to a length corresponding to the casing crown circumference, is likewise disposed circumferentially about the casing crown, overlying a layer of suitable bonding material, such as uncured cushion gum including a low temperature vulcanizing agent and accelerator. A roller pressing process, commonly referred to as stitching, is performed on the assembly to force out air from between the tread strip and casing. The assembly is then subjected to heat and pressure to cure (e.g., vulcanize) the bonding layer, generally in an autoclave. This bonds the prevulcanized tread strip to the casing, and typically, forces out any residual trapped air from between the tread and casing.
Perhaps the most common prevulcanized tread strip, known as a "top cap", is a strip of prevulcanized material, manifesting a predetermined tread design on the top surface thereof, and having an underside which is generally flat. Other top caps described in the prior art employ an underside having approximately the same curvature as the crown of the casing of the particular tire casing to be retreaded. The width of the top cap characteristically corresponds to the crown width of the casing. The characteristic shoulder trim angle for a casing is conventionally chosen to provide a substantially continuous interface at the juncture of the top cap and casing, i.e., to eliminate or reduce any ledge at the juncture, as well as to eliminate or reduce voids or remnants of the original tread design in the shoulder.
A number of problems arise from conventional trimming of the casing shoulder. If too much material is removed, for example, to eliminate or reduce voids or remnants of the original tread pattern in the shoulder, a narrow crown may result. Moreover, the integrity of the casing can be deleteriously affected by leaving insufficient material in the shoulder in the vicinity of end of the casing belt package. In such an instance, the casing tends to crack or otherwise break down in the shoulder area. A shoulder extrusion formed of uncured rubber stock can sometimes be disposed on the shoulder to, in effect, replace the shoulder area. However, such uncured rubber stock used in retreading tends to be relatively expensive for labor and material.
In addition, the buffed shoulder is often unsightly, even when structurally sound. This gives rise to a particular problem when the tire is used on, for example, a rental truck intended to be used by consumers who may not be familiar with retreads.
Precured tread strips having a contoured underside have been proposed. For example, a variant of the top cap including a substantially flat underside, but with tapering side lobes extending at an angle from the tread strip intended to overlie the shoulder of the casing is disclosed in Offenlegungsschrift 26 42 990 published on Apr. 7, 1977 in the name of Maltinti. The juncture between the flat underside and interior surface of the lobe is abrupt; the edges purportedly aid in centering the tread on the casing. Contrary to the teachings of Maltinti, the abrupt transition at the underside of the lobe causes the assembly to be susceptible to small offsets in alignment, and imprecisions in buffing the tire to the precise dimensions corresponding to the contour of the tread underside, causing air to be trapped between tread strip and casing, and potentially heat generating stresses in the tire.
Mechanisms that purport to provide a contoured tread that can be installed on a casing without creating stresses in the retread assembly have been suggested. For example, European Patent Application Publication No. 0 176 945 published in the name of Michelin Et Cie on Sep. 4, 1986 (bulletin 86/15) discloses a prevulcanized tread strip Which is curved both in the transverse and longitudinal (circumferential) directions. The radius of curvature of the tread underside in the transverse direction varies to better fit the shape of the top of the casing to be retreaded; the radius is maximum at the center and minimum at the shoulders. The curvature in the longitudinal (circumferential) direction purportedly avoids unacceptable stresses in the retread assembly.
U.S. Pat. No. 3,283,795 issued to Schelkmann on Nov. 8, 1966 discloses a prevulcanized tread strip having an underside manifesting a transverse curve with a radius of curvature greater than the radius of curvature of the casing crown. Such a contour is said to ensure that the central portion of the tread will first engage the casing crown, so that any air trapped between the tread strip and casing will be gradually squeezed out between the surfaces. Unacceptable stresses are purported to be avoided by employing a plurality of tread sections to form the retread assembly, each curved not only in the transverse direction, but also in the longitudinal direction (direction of elongation of the tread), or by including compensation folds or cuts in the tread.
It has also been proposed to avoid creating stresses in the retread assembly by employing, rather than a strip of tread, a premolded endless ring of tread having a relaxed diameter less than the inflated diameter of the tire casing. The replacement tread ring is stretched, disposed about the prepared tire casing, together with an intermediate layer of bonding material, then "snapped" onto the casing. The tread ring typically includes integral shoulder wings extending downwardly over the shoulders of the tire casing. An example of such a retreading system is described in U.S. Pat. No. 3,815,651 issued to Neal on Jun. 11, 1974.
However, prior art contoured treads are designed for a particular casing, and are relatively intolerant of varying crown arc width and radius; deviations from the specific crown width and crown radius create stresses in the retread assembly deleterious to operation and longevity. Thus, a large number of different tread strips are required to be on hand in order to recap the various commercially available tires.
There remains a need for a cold vulcanizing retreading process that substantially replicates the original tread width of the casing as a new tire, while at the same time accommodating a range of tire casings, e.g., a tire series, without creating deleterious stresses in the retread assembly. Moreover, there remains a need for a replacement tread, when used in such a process, which when applied to a tire casing forms a retread assembly having improved appearance and endurance, performance and wearability characteristics.